In the first report of this series (ib., Tetsu-to-Hagané, 45 (1959), No. 12, p. 1341) the effect of the grimary cooling on the billet surface was studied, and it was revealed that the improper condition of corner radius, uneven cooling, too-high casting temperature and too- high casting speed induced the longitudinal surface crack of billets. In this report, the mechanism of the evolution of this longitudinal crack on the surface of a continuous cast billet was investigated.
The mechanism of the evolution of cracks on the billet surface is essentially the same as that in the normal ingot casting. In continuous casting, however, because of the higher linear rate of casting, the tendency of the surface crack is greater than that in the normal ingot casting.
Ferrostatic pressure of the inner molten steel causes the bending stress in the outer solid shell at the time when the shell looses contact with mold. Formula of this stress σ which consists of ferrostatic pressure ω and thickness of solid shell d is as follows:
σ=3/16·ω·l (2x-l/3) /dd2
On the other hand, the strength of the shell depends upon the temperature.
Results of the experiment and the calculation reveal that the stress has a possibility of being greater than the strength. That is, the longitudinal crack may appear as soon as the billet looses contact with the mold.
The primary cooling conditions, corner radius, evenness of cooling, casting temperature and casting speed are all related with ω, d and temperature of the billet. And, the experimental results in the first report can be explained by this mechanism of the evolution of cracks.